Optical scanning apparatus utilizing a re-entrant laser beam

ABSTRACT

In the synchronous scanning display device disclosed, the laser active medium and means for mode-locking multiple-mode laser oscillations are disposed in a resonator adapted to provide a folded optical path yielding a large plurality of resolvable spots at a reflector of stepped transmissity and reflectivity. The intensity of the output from the resonator at each spot is selectively controlled by a video signal applied to a broad area modulator disposed just outside the stepped-reflectivity reflector. Typically, an isolator is employed to produce unidirectional propagation of the laser radiation along the folded path.

limited States Paten liaminow [451 Jan. 25, 1972 [541 OPTICAL SCANNINGAPPARATUS UTILIZING A RE-ENTRANT LASER BEAM [72] Inventor:

[73] Assignee:

Ivan P. Kaminow, New Shrewsbury, NJ.

Bell Telephone Laboratories, Incorporated, Murray Hill, BerkeleyHeights, NJ.

[22] Filed: Sept. 30, 1968 [21] Appl.No.: 763,778

[52] U.S. Cl ..l78/7.3, 178/76, 250/199, 331/945 [51] Int. Cl ..H04n3/02, HOls 3/08, I-I04b 9/00 [58] Field of Search ..178/7.3 D, 5.4 BD,7.88, 7.5 D, 178/76; 250/199, 229; 350/161, 160; 331/945 [56] ReferencesCited UNITED STATES PATENTS 3,303,276 2/1967 Haeff ..l78/5.4

3,488,102 1/1970 Buck et al ..178/7.6 3,437,954 4/1969 Herriott et al...33 1/94 5 3,497,826 2/1970 Foster 331/945 3,498,693 3/1970 Fein et al..l78/7.6 3,503,671 3/1970 Kompfner ..33l/94.5

Primary Examiner-Robert L. Griffin Assistant ExaminerJohn C. MartinAttorney-R. J. Guenther and Arthur J. Torsiglieri 57 ABSTRACT 5 Claims,2 Drawing Figures PARTIALLY lSOLATOR TRANSMISSIVE MIRROR DISPLAY SCREENL T \INTENSITY MODULATOR MODE-SPACING-RATE VIDEO SIGNAL MODULATOR SOURCEOPTICAL SCANNING APPARATUS UTILIZING A RE- ENTRANT LASER BEAM BACKGROUNDOF THE INVENTION This invention relates to optical scanners for coherentlight, particularly those in which the active light source is intimatelycombined with the scanning apparatus.

In some previously proposed optical scanners that would be I useful forscanning with coherent light, such as laser light, it has beenrecognized a single input optical pulse can be divided by a variety oftechniques into a plurality of optical pulses directed along distinctivepaths. Typically, these optical scanners have required a coherent lightsource, typically a pulsed laser, which is a self-contained unit thatmerely provides a pulsed output beam to the remainder of the scanningapparatus. A pulsed source has been found useful both for proposedoptical pulse-code-modulation (PCM) communication and for improvingresolution in some display applications.

I have recognized that substantially simpler scanning structures shouldbe possible. In particular, it would be desirable to reduce the numberof expensive optical reflectors required in the complete apparatus.

SUMMARY OF THE INVENTION In accordance with my invention, synchronousdisplay scanning is produced in a laser including means for modelockingmultiple-axial-mode oscillations by adapting the resonator to provide afolded optical path yielding a large number of resolvable spots at onereflector of the resonator. That one reflector is provided with gradedtransmissivity and reflectivity to pass substantially equal amplitudepulses from the spots along'distinct output paths. Typically, theresonator is adapted to provide the plurality of optical paths byoffsetting the laser active medium or media or the lowest loss pathbetween the reflectors, from the common axis of the resonatorreflectors. The reflectors are made much broader in area than thelateral dimensions of active media or the low loss path. Moreover, atleast one of the reflectors is curved to make a beam from the activemedium to be nonnormal at incidence on that reflector and to direct thesubsequent reflections of the beam in a reentrant folded path within theresonator.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of myinvention will become apparent from the following detailed descriptiontaken together with the drawing, in which:

FIG. 1 is a partially pictorial and partially block diagrammaticillustration of a first embodiment of the invention; and

FIG. 2 is a partially pictorial and partially block diagrammaticillustration of a modification of the embodiment of FIG. 1 in which thelow-loss path is defined by irises.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT In the illustrative embodiment ofthe drawing, the synchronous scanning laser 11 provides a spatial arrayof output light pulses in a repetitive sequence through graded (stepped)transmissivity areas of reflector 13 to a broad area intensity modulator14. The portions of the pulses transmitted through modulator 14 appearas an image or display on display screen 15. If desired, focusingdevices (not shown) may be employed between modulator 14 and screen 15to reduce the spot size at screen 15.

An amplitude-modulated signal is applied to modulator M from a suitablesignal source 16. Typically, the signal from source 16 may be of thetype used for modulating the display intensity in an ordinary televisionset.

The scanning laser 11 includes the curved, substantially totallyreflective reflector 12 as well as the graded transmissivity reflector13. The mean radius of curvature of astigmatic reflector 12 isillustratively 5,000 cm.; and the separation of reflectors 12 and 13 isillustratively 2,500 cm. In this case,

astigmatic reflector 12 is provided with a difference of radii ofcurvature in two orthogonal planes of a fraction of a centimeter. Thefacing surfaces of reflectors l2 and 13 have lateral dimensions of atleast 250 cm. in each of two mutually orthogonal coordinates and may beeither circular or square.

The laser 11 illustratively also includes two or more sections 17 and 18of active medium having dispositions between reflectors l2 and 113 toinsure that they intercept and are aligned with a common foldedreentrant ray path. The diameter of each is small enough to interceptonly a single beam segment. The lengths of the active medium sections 17and 18 depend inversely on their effective laser gain per unit lengthand are chosen to be sufficient to insure multiple-mode laseroscillation in view of the excitation provided by respective excitationsources 25 and 28 through electrodes 26, 27 and 29, 30, as shown.

To provide pulsing at the mode spacing rate, c/2NL, an optical amplitudeor phase modulator l9, energized and, operating at a frequency equal to,or a multiple of, that rate is disposed in a laterally displaced,peripheral segment of the folded ray path. The lateral displacement ofthis segment is provided for reasons of convenience in using areasonably sized modulator and is implemented by the planar, small area,obliquely disposed reflectors 20 and 21 intercepting the original raypath segment and by the reflectors 22 and 23 obliquely disposed withrespect to the displaced ray path to direct the light pulses throughmodulator 19 between reflectors 22 and 23.

Unidirectional traveling-wave oscillation in laser 11 is insured by anoptical isolator 24, which illustratively may comprise a 45 Faradaypolarization-rotator disposed between crossed polarizers. The activepolarization-rotating material in isolator 24 might illustratively belead glass, which is suitable for use with helium-neon laser sections 17and 18 operating at 6,328 A. (l A., Angstrom unit, equals 10 microns or10' centimeters).

The active material of modulator 19 may be lithium tantalate employedwith laterally disposed electrodes normal to its Z crystalline axis. Thelight is transmitted along the X crystalline axis. In this illustrativecase the linear electro-optic effect is employed; and amplitudemodulation is provided by disposing the active material between crossedpolarizers. The signal source for modulator 19 is assumed to be a partthereof for the purpose of illustration and obviously would be connectedbetween the electrodes.

In the formula for the mode-spacing rate c/2NL, c is the velocity oflight, L is the length of the resonator 11 and 2N is the number ofsegments in the folded ray path. In a practical laser II, N will be ofthe order of several thousand; and the angles between ray path segmentswill be typically much smaller than those illustrated.

The large-area intensity modulator 14 may be a wide-area modulator or amosaic of lithium tantalate electro-optic cells, each aligned with oneof the output paths from laser 11. The entire mosaic of electro-opticcells is followed by a large-area polarizer to complete intensitymodulator 14.

All of the polarizers in the components described above are mutuallyoriented to provide minimum attenuation for a light pulse propagating inthe selected sense in laser 11 in the absence of signals applied tomodulators l9 and 14.

The display screen 15 is illustratively of the ground-glass typeconventional in the optical art.

The graded reflectivity of reflector 13 is provided by stepwise changesof the reflectivity and of the transmissivity of essentially rectangularareas thereon somewhat larger than the spot size of the incidentcoherent light pulse. The step changes in reflectivity andtransmissivity vary monotonically in the same sequence as the sequenceof incidences of the light pulse and are calculated to provide thatequal-intensity portions of the pulse are transmitted through reflector13 at each incidence. It may be noted that adjacent areas can differ byseveral steps of reflectivity, depending on the scanning patternpreferred by the attitudes of the active medium sections 17 and 18.

Various techniques are well known in the optical art for treating areflector to provide selected reflectivity and transmissivitycharacteristics. Preparatory to use of any of these techniques, thesequence of reflectivity steps for the reflector are mapped and thevalues of the reflectivity steps are calculated to insure maintenance ofoscillations and equal spot intensities. The calculations arestraightforward.

In the operation of the embodiment of the drawing, the cooperation ofreflectors l2 and 13 with the particularly disposed active mediumsections 17 and 18, the modulator 19 and the isolator 24 is to promotemultiple-mode unidirectional traveling-wave oscillation along the foldedmultiple-segment ray path passing through both active media sections.The path must be reentrant so that it closes on itself and retracesitself, in order for oscillations to occur. This adaptation alsoprovides fixed disposition of the displayed spots on screen 15.

The amplitude modulation provided by modulator l9 mode-locks themultiple axial modes, thereby providing sequential pulses at themode-spacing rate at spatially separated locations.

The display information is imparted to the sequentially incident pulsesby modulator 14 in a sequence that appropriately corresponds to thescanning sequence of laser 11. Thus, the signal from source 16 variessequentially between values corresponding to the desired intensities atareas of screen 15 upon which are imaged the areas of successivelydecreasing reflectivity of reflector 13.

The scanning period of the apparatus of the drawing is approximately thereciprocal of the above-described mode spacing rate. For N=500X500=2.5 lspots, the scanning period is about one twenty-fifth of a second, whichis appropriate for television scanning.

Several possible modifications are within the spirit of my invention.For example, a scanning laser according to my invention is readilyadapted for multiple-channel optical pulse-code modulation (PCM)communication by replacing modulator 14 with N separated modulatorsintercepting respective sequential output pulses and responding to therespective information signal sources of the different channels. Displayscreen would be replaced by conventional arrays of partiallytransmissive reflectors for recombining the separate pulse streams ofthe separate channels for transmission in a common transmission path.

Moreover, in embodiments intended for display, screen 15 could bereplaced by any other means for displaying some function of the relativeintensities of the pulses. For example, reversible dyes exhibiting twodistinct observable states could be employed. Such embodiments can beadapted for information storage.

In the modified embodiment of FIG. 2, components like those of FIG. 1are numbered with the same numbers as in FIG. 1. The multiple sectionsof active medium are replaced by the broad area active medium 37disposed between reflectors l2 and I3. In the case of the use ofhelium-carbon dioxide mixtures operating at 10.6 microns, such largetube diameters are practical, at the sacrifice'of some gain. The displayscreen 15 may be an infrared-responsive fluorescent screen. The folded,reentrant, oscillation path is now defined by apertures in the screen38, which is illustratively disposed near reflector 12 so that nooscillation can occur in a path shorter than that desired. For ahelium-carbon dioxide active medium in the tube 37, the opticalmaterials of components 24, 19 and 14 would be chosen to be suitable.The embodiment of FIG. 2 operates in essentially the same manner, inother respects, as the embodiment of FIG. 1.

[claim 1. An optical scanning apparatus comprising a folded opticalresonator including first and second reflectors, one of said reflectorsbeing partially transmissive, a section of a laser active mediumdisposed between said reflectors in said resonator, said resonator withsaid medium being adapted to define between said reflectors a foldedreentrant optical path having successive spots of incidence upon saidpartially transmissive reflector in a selected scanning se uence, andmeans disposed outside of said resonator for uti lzmg radiation emittedin sequence through said spots.

2. An apparatus according to claim 1 including means for energizing saidactive medium to enable coherent oscillations, said active medium andsaid energizing means being mutually adapted for promoting oscillationsin multiple axial modes, and including means for mode-locking saidoscillations, whereby pulses are emitted in succession throughrespective different ones of said spots on said partially transmissivereflector.

3. An apparatus according to claim 2 including optical isolation meansdisposed in said ray path in said resonator for promoting unidirectionalpropagation of said multiple modes, and in which the utilizing meanscomprises means disposed to intercept the emitted pulses of radiationfor displaying an effect of their relative intensities and meansdisposed between the partially transmissive reflector and the displayingmeans for modulating the intensities of successive pulses.

4. An apparatus according to claim 1 in which the medium and theresonator are adapted to define the path in that the medium includes aplurality of sections offset from the axis of the resonator and alignedwith different segments of the folded path.

5. An apparatus according to claim 1 in which the medium and theresonator are adapted to define the path in that the medium intercepts aplurality of segments of said folded path as well as the resonator axis,and the resonator includes a member apertured for the segments of thefolded path and blocking oscillation in any path shorter than the foldedpath.

1. An optical scanning apparatus comprising a folded optical resonatorincluding first and second reflectors, one of said reflectors beingpartially transmissive, a section of a laser active medium disposedbetween said reflectors in said resonator, said resonator with saidmedium being adapted to define between said reflectors a foldedreentrant optical path having successive spots of incidence upon saidpartially transmissive reflector in a selected scanning sequence, andmeans disposed outside of said resonator for utilizing radiation emittedin sequence through said spots.
 2. An apparatus according to claim 1including means for energizing said active medium to enable coherentoscillations, said active medium and said energizing means beingmutually adapted for promoting oscillations in multiple axial modes, andincluding means for mode-locking said oscillations, whereby pulses areemitted in succession through respective different ones of said spots onsaid partially transmissive reflector.
 3. An apparatus according toclaim 2 including optical isolation means disposed in said ray path insaid resonator for promoting unidirectional propagation of said multiplemodes, and in which the utilizing means comprises means disposed tointercept the emitted pulses of radiation for displaying an effect oftheir relative intensities and means disposed between the partiallytransmissive reflector and the displaying means for modulating theintensities of successive pulses.
 4. An apparatus according to claim 1in which the medium and the resonator are adapted to define the path inthat the medium includes a plurality of sections offset from the axis ofthe resonator and aligned with different segments of the folded path. 5.An apparatus according to claim 1 in which the medium and the resonatorare adapted to define the path in that the medium intercepts a pluralityof segments of said folded path as well as the resonator axis, and theresonator includes a member apertured for the segments of the foldedpath and blocking oscillation in any path shorter than the folded path.